The search for new therapeutic agents has been greatly aided in recent years by a better understanding of the structure of enzymes and other biomolecules associated with diseases. One important class of enzymes that has been the subject of extensive study is protein kinases.
Protein kinases constitute a large family of structurally related enzymes that are responsible for the control of a variety of signal transduction processes within the cell. (See, Hardie, G. and Hanks, S. The Protein Kinase Facts Book, I and II, Academic Press, San Diego, Calif.: 1995). Protein kinases are thought to have evolved from a common ancestral gene due to the conservation of their structure and catalytic function. Almost all kinases contain a similar 250-300 amino acid catalytic domain. The kinases may be categorized into families by the substrates they phosphorylate (e.g., protein-tyrosine, protein-serine/threonine, lipids, etc.). Sequence motifs have been identified that generally correspond to each of these kinase families (See, for example, Hanks, S. K., Hunter, T., FASEB J. 1995, 9, 576-596; Knighton et al., Science 1991, 253, 407-414; Hiles et al., Cell 1992, 70, 419-429; Kunz et al., Cell 1993, 73, 585-596; Garcia-Bustos et al., EMBO J. 1994, 13, 2352-2361).
In general, protein kinases mediate intracellular signaling by effecting a phosphoryl transfer from a nucleoside triphosphate to a protein acceptor that is involved in a signaling pathway. These phosphorylation events act as molecular on/off switches that can modulate or regulate the target protein biological function. These phosphorylation events are ultimately triggered in response to a variety of extracellular and other stimuli. Examples of such stimuli include environmental and chemical stress signals (e.g., osmotic shock, heat shock, ultraviolet radiation, bacterial endotoxin, and H2O2), cytokines (e.g., interleukin-1 (IL-1) and tumor necrosis factor α (TNF-α)), and growth factors (e.g., granulocyte macrophage-colony-stimulating factor (GM-CSF), and fibroblast growth factor (FGF)). An extracellular stimulus may affect one or more cellular responses related to cell growth, migration, differentiation, secretion of hormones, activation of transcription factors, muscle contraction, glucose metabolism, control of protein synthesis, and regulation of the cell cycle.
Many diseases are associated with abnormal cellular responses triggered by protein kinase-mediated events as described above. These diseases include, but are not limited to, autoimmune diseases, inflammatory diseases, bone diseases, metabolic diseases, neurological and neurodegenerative diseases, cancer, cardiovascular diseases, allergies and asthma, Alzheimer's disease, and hormone-related diseases. Accordingly, there has been a substantial effort in medicinal chemistry to find protein kinase inhibitors that are effective as therapeutic agents.
Syk is a tyrosine kinase that plays a critical role in FcεRI mediated mast cell degranulation and eosinophil activation. Accordingly, Syk kinase is implicated in various allergic disorders, in particular asthma. It has been shown that Syk binds to the phosphorylated gamma chain of the FcεR1 receptor via N-terminal SH2 domains and is essential for downstream signaling [Taylor et al., Mol. Cell. Biol. 1995, 15, 4149].
Inhibition of eosinophil apoptosis has been proposed as a key mechanism for the development of blood and tissue eosinophilia in asthma. IL-5 and GM-CSF are upregulated in asthma and are proposed to cause blood and tissue eosinophilia by inhibition of eosinophil apoptosis. Inhibition of eosinophil apoptosis has been proposed as a key mechanism for the development of blood and tissue eosinophilia in asthma. It has been reported that Syk kinase is required for the prevention of eosinophil apoptosis by cytokines (using antisense)[Yousefi et al., J. Exp. Med. 1996, 183, 1407].
The role of Syk in FcγR dependent and independent response in bone marrow derived macrophages has been determined by using irradiated mouse chimeras reconstituted with fetal liver cells from Syk −/− embryos. Syk deficient macrophages were defective in phagocytosis induced by FcγR but showed normal phagocytosis in response to complement [Kiefer et al., Mol. Cell. Biol. 1998, 18, 4209]. It has also been reported that aerosolized Syk antisense suppresses Syk expression and mediator release from macrophages [Stenton et al., J. Immunology 2000, 164, 3790].
ZAP-70 is essential for T-cell receptor signalling. Expression of this tyrosine kinase is restricted to T-cells and natural killer cells. The importance of ZAP-70 in T-cell function has been demonstrated in human patients, human T-cell lines and mice. Human patients suffering from a rare form of severe combined deficiency syndrome (SCID) possess homozygous mutations in ZAP-70 (reviewed in Elder J. of Pedriatric Hematology/Oncology 1997, 19(6), 546-550). These patients have profound immunodeficiency, lack CD8+ T-cells and have CD4+ T-cells that are unresponsive to T-cell receptor (TCR)-mediated stimulation. Following TCR activation these CD4+ cells show severe defects in Ca2+ mobilization, tyrosine phosphorylation of down-stream substrates, proliferation and IL-2 production 70 (reviewed in Elder Pedriatric Research 39, 743-748). Human Jurkat cells lacking ZAP-70 also provide important insights into the critical role of ZAP-70 in T-cell receptor signalling. A Jurkat clone (p116) with no detectable ZAP-70 protein was shown to have defects in T-cell receptor signalling which could be corrected by re-introduction of wild type ZAP-70 (Williams et al., Molecular and Cellular Biology 1998, 18 (3), 1388-1399). Studies of mice lacking ZAP-70 also demonstrate a requirement of ZAP-70 in T-cell receptor signalling. ZAP-70-deficient mice have profound defects in T-cell development and T-cell receptor signalling in thymocytes is impaired (Negishi et al., Nature 1995 376, 435-438).
The importance of the kinase domain in ZAP-70 function is demonstrated by studies of human patients and mice expressing identical mutations in the DLAARN motif within the kinase domain of ZAP-70. Inactivation of kinase activity by this mutation results in defective T-cell receptor signalling (Elder et al., J. Immunology 2001, 656-661). Catalytically inactive ZAP-70 (Lys369Arg) was also defective in restoring T-cell receptor signalling in a ZAP-70 deficient Jurkat cell clone (p116) (Williams et al., Molecular and Cellular Biology 1998, 18 (3), 1388-1399).
Accordingly, there is a great need to develop compounds useful as inhibitors of protein kinases. In particular, it would be desirable to develop compounds that are useful as inhibitors of SYK or ZAP-70, particularly given the inadequate treatments currently available for the majority of the disorders implicated in their activation.